There has been known a slope A/D converter which achieves A/D conversion of an image sensor by comparing an image signal with a reference voltage having a ramp waveform by use of a comparator, and counting a time until inversion of output from the comparator. The slope A/D converter is excellent in linearity and noise characteristics. Further, there has also been a column A/D converter which includes a plurality of A/D converters for each pixel column to simultaneously achieve A/D conversion of each pixel column. The column A/D converter realizes speed increase by decreasing an operation frequency per one A/D converter, and improves area efficiency and power consumption efficiency by providing a reference voltage generator shared by the respective A/D converters. Accordingly, the column A/D converter is more compatible with a complementary metal oxide semiconductor (CMOS) image sensor than other types of A/D conversion systems.
There has been considered a method of the foregoing slope A/D conversion which selects a slope reference signal from slope reference signals having different inclinations on the basis of level determination of pixel signals to obtain similar effects by using a column-parallel A/D conversion circuit (for example, see Patent Document 1 and Patent Document 2).
On the other hand, there has been considered a method of the foregoing column A/D conversion which obtains lower bits by using a gray code counter and a latch provided for each of a plurality of columns, and higher bits by using a binary ripple counter provided for each of the columns to reduce power consumption (for example, see Patent Document 3 and Patent Document 4).
Incidentally, for A/D conversion of pixel signals of an image sensor, there has been a method which reduces kTC noise or the like contained in pixel signals by using correlated double sampling (CDS) to prevent lowering of image quality of a captured image.